[YSQL] It is not safe to call YBOnPostgresBackendShutdown from quickdie

[d-uspenskiy]

Jira Link: DB-2635

Description

The quickdie function is called as a signal handler. As a result this function should call async-signal-safe functions only.
The YBOnPostgresBackendShutdown function is too complex to be called from signal handler as this function destroys the pggate::PgApiImpl object and the destructor of this object locks mutexes.

Also it is not safe to destroy the pggate::PgApiImpl object because YSQL is a multi-threaded process. As a result quickdie can be called from any of the thread. As a result pggate::PgApiImpl object can be destroyed while it is still in use by the YSQL main thread.

Example:

YSQL's main thread calls YBCInsertSequenceTuple function

...
if (IsYugaByteEnabled())
{
    HandleYBStatus(YBCInsertSequenceTuple(...)); // This function uses the `pggate::PgApiImpl` object
}
...

at same time another thread handles SIGQUIT signal by calling quickdie.

As a result it is possible that quickdie will delete the pggate::PgApiImpl object while it is still in use.

[mbautin]

Can we set a flag or enqueue a task in the signal handler that will then cause shutdown to happen in a regular thread?